This invention relates to a rechargeable electrochemical cell and more particularly to a rechargeable electrochemical cell having improved means for providing an electrical connection between one of the electrodes in the cell and the container in which the cell electrode assembly resides.
In rechargeable electrochemical cells, such as nickel-cadmium cells, it is necessary to establish an electrical connection between the external terminals of the cell and the electrodes contained within the cell. In some cell designs, the cell container functions as one of the external terminals of the cell and accordingly electrical connection must be provided between the container and one of the electrodes within the cell. Establishing an effective reliable connection is considerably important since the rate of discharge of electrical energy the cell is directly dependent upon the integrity of the connection.
The connection between the container and the electrode serves two functions. First, the connection must act as a collector of current from the electrode to which it is connected and, second, the connection must function to deliver the current to the cell container. If either of these functions are not adequately performed the cell will exhibit a high resistance to the passage of current and attendant low discharge rates and low output voltages.
In the prior art, many different approaches have been pursued in attempts to provide suitable current collectors. In one such approach, of which U.S. Pat. Nos. 3,508,806, 3,732,124, and 4,452,869 are typical, a current collector is inserted into the cell container between the spirally wound electrode assembly and the bottom of the container. The electrode assembly is comprised of positive and negative electrodes which are axially offset with respect to each other so that the edge of only one electrode may be contacted by one current collector. The current collector is provided with one or more rigid projections adapted to engage the axially offset edge of the electrode. The rigid projections are welded to the offset edge of the electrode and hence are adapted to collect current from the cell electrode. In order to deliver the collected current to the cell container, the current collector may be provided with an integrally formed strap which may be welded to the bottom of the cell container.
The approach described in the aforementioned patents has been commonly termed the edge-welded approach since the rigid projections of the current collector are welded to the offset edge of one of the cell electrodes. Use of a welding procedure to effect electrical contact between the rigid projections of the current collector and the electrode is not entirely suitable in a high volume production line. Welding of the rigid projections of the collector to the electrode time consuming and costly. Furthermore, subsequent welding of the collector to the cell container after insertion of the electrode assembly into the container is cumbersome and may result in an undetected partial or totally incomplete weld.
In an attempt to overcome some of these manufacturing and assembly problems, one approach has eliminated the weld operation affixing the strap of the collector to the cell container. Instead, electrical contact has been achieved by folding the aforementioned strap between the outer cylindrical surface of the electrode assembly and the inner cylindrical surface of the cell container as exemplified by U.S. Pat. No. 4,452,869. However, under high rate production conditions proper folding of the strap may not always occur and sufficient electrical connection may not be achieved. In another attempt to overcome the aforementioned difficulties, U.S. Pat. No. 4,009,053 suggests that, while welding is preferred, the connection of the rigid projections of the current collector to the electrode may be accomplished by pressure. However, with variations in cell component dimensions, even variations well within normal and acceptable tolerances, pressure contact between the rigid projections of the current collector and the edge of the electrode may not always be assured under conditions of high rate manufacture. Furthermore, vibration or impact forces upon the cell during its useful life may shift the electrode assembly within the container whereby the electrical contact is broken between the rigid projections of the current collector and the electrode. The present invention addresses these problems and difficulties in a new and novel manner.
It is therefore an object of the present invention to provide a rechargeable electrochemical cell having an improved current collector for providing an electrical path from one of the cell electrodes to one of the external terminals of the cell.
It is another object of the present invention to provide a rechargeable electrochemical cell having a current collector which achieves sustained electrical contact with the cell electrode without welding of the collector to the edge of the electrode.
It is yet another object of the present invention to provide a rechargeable electrochemical cell having a current collector which is welded to the cell container prior to insertion of the electrode assembly into the cell container.
It is still another object of the present invention to provide a rechargeable electrochemical cell having a current collector configuration which will reliably and invariably establish a sufficient electrical path between the cell electrode and the external terminal of the cell even under conditions associated with high rate manufacturing assembly production lines and under conditions wherein the cell may be subjected to severe impact or vibration.